How to make object eligible for garbage collection in Java?

An object is eligible to be garbage collected if its reference variable is lost from the program during execution.Sometimes they are also called unreachable objects.

What is reference of an object?

The new operator dynamically allocates memory for an object and returns a reference to it. This reference is the address in memory of the object allocated by new. A reference is an address that indicates where an object’s variables, methods etc. are stored.

The objects are not actually used when assigned to a variable or passed as an argument to a method . The references to objects are used everywhere. Example:

Box mybox =  new Box();   //referencing to object

Role of an unreachable objects in java

In java, the memory allocated at runtime i.e. heap area can be made free by the process of garbage collection. It is nothing but just a method of making the memory free which is not being used by the programmer. Only the objects who have no longer reference to them are eligible for garbage collection in java.

Ways to make an object eligible for garbage collection:

Please note that the object can not become a candidate for garbage collection until all references to it are discarded.

1. Object created inside a method : When a method is called it goes inside the stack frame. When the method is popped from the stack, all its members dies and if some objects were created inside it then these objects becomes unreachable or anonymous after method execution and thus becomes eligible for garbage collection
   .Example:

   
   

   
   
   

   /* Java program to demonstrate that  objects created inside a method will becomes eligible for gc after method execution terminate */    class Test {            // to store object name     String obj_name;            public Test(String obj_name)      {         this.obj_name = obj_name;     }            static void show()     {         //object t1 inside method becomes unreachable when show() removed         Test t1 = new Test("t1");          display();                }     static void display()     {         //object t2 inside method becomes unreachable when display() removed         Test t2 = new Test("t2");      }            // Driver method     public static void main(String args[])     {         // calling show()         show();                    // calling garbage collector         System.gc();     }            @Override     /* Overriding finalize method to check which object     is garbage collected */     protected void finalize() throws Throwable      {         // will print name of object         System.out.println(this.obj_name + " successfully garbage collected");     } }

   Output:

   t2 successfully garbage collected
   t1 successfully garbage collected
   

   Note : If a method returns the object created inside it and we store this object reference by using a reference-type variable than it is no longer eligible for garbage collection.

2. Reassigning the reference variable: When reference id of one object is referenced to reference id of some other object then the previous object has no any longer reference to it and becomes unreachable and thus becomes eligible for garbage collection.Example:

   
   

   
   
   

   /* Java program to demonstrate gc  when one object referred to other object */    class Test {     // to store object name     String obj_name;            public Test(String obj_name)      {         this.obj_name = obj_name;     }            // Driver method     public static void main(String args[])     {         Test t1 = new Test("t1");         Test t2 = new Test("t2");                    //t1 now referred to t2         t1 = t2;                    // calling garbage collector         System.gc();     }            @Override     /* Overriding finalize method to check which object      is garbage collected */     protected void finalize() throws Throwable      {         // will print name of object         System.out.println(this.obj_name + " successfully garbage collected");     } }

   Output:

   t1 successfully garbage collected
   

3. Nullifying the reference variable : When all the reference variables of an object are changed to NULL, it becomes unreachable and thus becomes eligible for garbage collection.Example:

   
   

   
   
   

   /* Java program to demonstrate gc  when object reference changed to NULL */    class Test {     // to store object name     String obj_name;            public Test(String obj_name)      {         this.obj_name = obj_name;     }            // Driver method     public static void main(String args[])     {         Test t1 = new Test("t1");                     /* t1 being used for some purpose in program */             /* When there is no more use of t1, make the object            referred by t1 eligible for garbage collection */                 t1 = null;             // calling garbage collector         System.gc();     }            @Override     /* Overriding finalize method to check which object      is garbage collected */     protected void finalize() throws Throwable      {         // will print name of object         System.out.println(this.obj_name + " successfully garbage collected");     } }

   Output:

   t1 successfully garbage collected
   

4. Anonymous object : The reference id of an anonymous object is not stored anywhere. Hence, it becomes unreachable.
   Example:

   
   

   
   
   

   /* Java program to demonstrate gc  of anonymous objects */    class Test {     // to store object name     String obj_name;            public Test(String obj_name)      {         this.obj_name = obj_name;     }            // Driver method     public static void main(String args[])     {         //anonymous object without reference id         new Test("t1");              // calling garbage collector         System.gc();     }            @Override     /* Overriding finalize method to check which object      is garbage collected */     protected void finalize() throws Throwable      {         // will print name of object         System.out.println(this.obj_name + " successfully garbage collected");     } }

   Output:

   t1 successfully garbage collected
   

Related Article: Island of Isolation

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